Clean up some old and obsolete TODOs of mine (#81631)

packages/flutter/lib/src/material/theme.dart

@@ -221,7 +221,6 @@ class _AnimatedThemeState extends AnimatedWidgetBaseState<AnimatedTheme> {
 
   @override
   void forEachTween(TweenVisitor<dynamic> visitor) {
-    // TODO(ianh): Use constructor tear-offs when it becomes possible, https://github.com/dart-lang/sdk/issues/10659
     _data = visitor(_data, widget.data, (dynamic value) => ThemeDataTween(begin: value as ThemeData))! as ThemeDataTween;
   }
 

packages/flutter/lib/src/rendering/error.dart

@@ -56,8 +56,7 @@ class RenderErrorBox extends RenderBox {
   /// The message to attempt to display at paint time.
   final String message;
 
-  // TODO(ianh): should be final
-  ui.Paragraph? _paragraph;
+  late final ui.Paragraph? _paragraph;
 
   @override
   double computeMaxIntrinsicWidth(double height) {

packages/flutter/lib/src/widgets/async.dart

@@ -485,7 +485,6 @@ typedef AsyncWidgetBuilder<T> = Widget Function(BuildContext context, AsyncSnaps
 ///    [Stream].
 ///  * [StreamBuilderBase], which supports widget building based on a computation
 ///    that spans all interactions made with the stream.
-// TODO(ianh): remove unreachable code above once https://github.com/dart-lang/linter/issues/1139 is fixed
 class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
   /// Creates a new [StreamBuilder] that builds itself based on the latest
   /// snapshot of interaction with the specified [stream] and whose build